`chain-spec-builder`: info about patch/full files added (#6373)

There was no good example of what is patch and full genesis config file.
Some explanation and example were added to the `chain-spec-builder` doc.

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Iulian Barbu <14218860+iulianbarbu@users.noreply.github.com>